Seismic Anisotropy And Time-Frequency Analyses During Taupō's 2019 Unrest

The currently active Taupō volcano (with its most recent eruptive event at ∼ 1.8 ka) was responsible for Earth’s youngest supereruption at 25.5 ka. In the past 141 years, 21 unrest events have been observed in records of seismicity, crustal deformation and changes in the lake level at Taupō. The most recent unrest was during September 2018 – September 2019, and was interpreted to be caused by a magmatic intrusion rather than by hydrothermal instabilities in the volcano (Illsley-Kemp et al., 2021). Prior seismic anisotropy studies in the North Island have identified a distinct pattern of S-wave fast orientations at Taupō volcano that do not align with the maximum horizontal stress in the region. Therefore, due to the complexity of the unrest of the caldera, we combined seismic anisotropy and time-frequency analysis to complement the current understanding of volcano dynamics over the period of unrest. We used an existing 10-year earthquake catalogue determined by Illsley-Kemp et al. (2021) to study seismic anisotropy around Lake Taupō, with a focus on temporal analysis during the unrest period. Using 35,452 high quality shear-wave splitting measurements, we obtained the median fast orientation values for 23 seismic stations around Taupō. We recognised three significant anisotropy temporal changes during the unrest period, not related to variations in the travel path of the earthquakes, and interpreted these as stress-induced changes due to magmatic processes, in accordance with the findings of Illsley-Kemp et al. (2021). Temporal stress-changes identified by seismic anisotropy were compared with the results of time-frequency analysis to determine that the magmatic intrusion did not reach shallower depths.

We reviewed the continuous stream of seismic data in the time-frequency domain to identify possible low frequency signals related to volcanic activity and to compare their relationship with stress changes during the unrest period. Even though we did not identify low frequency volcanic seismic signals during 2019, we propose a preliminary catalogue of typical low frequency seismic signals at Taupō, which includes the lake microseism signal. Volcanic unrest in calderas is a frequently occurring phenomenon, and does not always imply a volcanic eruption. The improvement of monitoring techniques will allow us to understand periods of unrest in calderas for volcano preparedness and risk management.